1. Field of the Invention
The present invention relates to a computer-implemented method and system for designing a workcell layout.
2. Background Art
Currently, workcell layout design is a cumbersome and time-consuming process. It is either performed manually with paper and pencil or with a non-interactive, two-dimensional computer interface. One disadvantage of this methodology is that every time a change is made in the sequence of workcell tasks, or any of the workcell contents are moved, the layout design typically has to be redone from scratch and completely recalculated. Another disadvantage of the conventional methodology is that it is typically only performed by a small group of plant engineers who are responsible for optimizing the efficiency of potentially hundreds of individual workcells and operators. This method is particularly inefficient because the operators themselves often know best what workcell layout(s) may increase the efficiency of their work and, consequently, the quality of the products. It should also be noted that for certain operations requiring ergonomic assessment, there exist detailed three-dimensional computer-aided design (CAD) representations of an operator workcell. However, these tools do not directly address the problem of increasing the efficiency of a workcell layout, vis-a-vis, the moving vehicles being assembled.
As a whole, the situation and methods outlined above do not satisfactorily address the problem of workcell design and planning in a moving assembly line for at least three reasons. First, when performed in this way, the planning process is cumbersome and time consuming. Second, the tools that have been used until now are static and suitable only for engineering planning: they do not support real-time line operator interaction. Third, since the process is largely manual, it is not easy for industrial engineers to uncover the layout that will produce the shortest operator walking path. In other words, inefficient choices (e.g., choices forcing the operator to needlessly spend time walking) may be made. In this regard, there are many factors that may complicate the choice of the best arrangement. Just to cite a few: for each workcell, there are several bins of parts and tools that need to be positioned; there may be external constraints (e.g., fixed obstacles, such as pillars located within the workcell); the operator moves at a speed that changes depending on the weight of the part he/she is carrying; the assembly line may be building a certain vehicle mix (e.g., two or three different vehicle models on the same line: then, at each workstation, different tasks may have to be performed depending on which vehicle is moving through).
In spite of research in the area of moving assembly line design and of line balancing, the practical application of published algorithms is limited. This is because of two main reasons. First, the published algorithms are not sufficiently flexible to model the actual conditions of assembly line workstations. Second, accurate data is difficult to obtain and maintain: often, there is no current record of a workcell layout; that is, when a workcell layout is modified the corresponding central records are not updated.
In the manual “pencil and paper approach”, also known as the “wall process” (illustrated in FIG. 1), the workcell is divided into squares, and time is divided into discrete time steps. A certain number of time steps (dependent on the operator velocity) is required for the operator to move across a square. On the paper, a transparent vehicle is moved in discrete steps to determine the intersections between the moving car (on a moving assembly line) and the operator's walk path. This manual approach can be very tedious because of the large amount of manual calculations required for each path and workcell. When any of the part bins are rearranged or the line speed is changed, the calculation must be repeated to obtain the new total time. It is not surprising that this method often fails to produce an efficient workcell arrangement. The fact that the method is used for each workcell in the assembly line compounds time inefficiencies.
The two-dimensional tool approach uses an animation of the actual sequence of work elements in an operation: input information includes rack layouts, vehicle speed and configurations, task description and times (including walk times) associated with a task. Based on this input information the two dimensional computer based tool allows the user to see where an operator would perform a given task. While this approach represents an improvement over the paper and pencil method, there is no attempt to compute on the fly walking time based on the layout of the workcell, nor is there any attempt to systematically optimize workcell configuration.